Officials had expressed cautious optimism that the missile would hit the satellite, which was the size of a school bus. But they were less certain of hitting the smaller, more problematic fuel tank, whose contents posed what Bush administration officials deemed a potential health hazard to humans if it landed intact.

In a statement late Wednesday announcing that the Navy missile struck the satellite, the Pentagon said, “Confirmation that the fuel tank has been fragmented should be available within 24 hours.” It made no mention of early indications, but a defense official close to the situation said later that officials monitoring the collision saw what appeared to be an explosion, indicating that the fuel tank was hit.

The USS Lake Erie, armed with an SM-3 missile designed to knock down incoming missiles - not orbiting satellites - launched the attack at 10:26 pm EST Wednesday (0326 GMT Thursday), according to the Pentagon. It hit the satellite about three minutes later as the spacecraft traveled in polar orbit at more than 17,000 mph (27,000 kilometres per hour).

Because the satellite was orbiting at a relatively low altitude at the time it was hit by the missile, debris will begin to re-enter the Earth’s atmosphere immediately, the Pentagon statement said.

“Nearly all of the debris will burn up on re-entry within 24-48 hours and the remaining debris should re-enter within 40 days,” it said.

The use of the Navy missile amounted to an unprecedented use of components of the Pentagon’s missile defense system, designed to shoot down hostile ballistic missiles in flight - not kill satellites.

The operation was so extraordinary, with such intense international publicity and political ramifications, that Defense Secretary Robert Gates, rather than a military commander, was to make the final decision to pull the trigger.

The government organised hazardous materials teams, under the code name “Burnt Frost,” to be flown to the site of any dangerous or otherwise sensitive debris that might land in the United States or elsewhere.

Also, six federal response groups that are positioned across the country by the Federal Emergency Management Agency have been alerted but not activated, FEMA spokesman James McIntyre said. “These are purely precautionary and preparedness actions only,” he said. (AP)